In a typical aircraft/ground station voice communication system, a controller located in a tower initiates voice transmission when he closes his key (presses the push-to-talk switch on his microphone). The communication is received by the radio station onboard an aircraft to provide information and instructions to the pilot of the aircraft.
During the voice communications from the controller to a pilot in the aircraft, it is presumed by the controller that the aircraft pilot is receiving and listening to his transmission. With the widespread use of squelch in receivers, there is no direct method now available for the pilot to determine that the message has been completed. The use of the term "over and out" is no longer practiced. Because of the vagaries of the language and the potential for misunderstanding on the part of the pilot as to when a message has ended, the voice procedures practice is simply not trustworthy. Therefore, it is possible that while the tower controller is continuing his transmission of further instructions to the aircraft, the pilot may himself pick up his own microphone and press his push-to-talk switch to start transmitting. On these occasions the pilot would be transmitting simultaneously during the time that the tower is transmitting.
The condition that may be created is that the tower may not know that the aircraft is not receiving that portion of the tower originated message transmitted during simultaneous transmission; or any of it, and the pilot of the aircraft does not know that there has been additional instructions or clearance information given to him while he was transmitting simultaneously with the tower.